Re: [PATCH] x86,seccomp,prctl: Remove PR_TSC_SIGSEGV and seccomp TSC filtering

[Andy Lutomirski]

On Fri, Oct 3, 2014 at 2:02 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Fri, Oct 03, 2014 at 10:44:43PM +0200, Peter Zijlstra wrote:
>> On Fri, Oct 03, 2014 at 01:27:52PM -0700, Andy Lutomirski wrote:
>> > On Fri, Oct 3, 2014 at 1:22 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> > >
>> > > We could make the rule be that RDPMC is enabled if a perf event is
>> > > mmapped or TIF_SECCOMP is clear, but I'd prefer to be convinced that
>> > > there's an actual performance issue first.  Ideally we can get this
>> > > all working with no API or ABI change at all.
>> >
>> > No, we can't use that rule.  But we could say that RDPMC is enabled if
>> > a perf event is mmapped and no thread in the mm uses seccomp.  I'll
>> > grumble a little bit about adding yet another piece of seccomp state.
>>
>> Well, we could simply disable the RDPMC for everything TIF_SECCOMP.
>> Should be fairly straight fwd.
>
>
> Something like so.. slightly less ugly and possibly with more
> complicated conditions setting the cr4 if you want to fix tsc vs seccomp
> as well.

This will crash anything that tries rdpmc in an allow-everything
seccomp sandbox.  It's also not very compatible with my grand scheme
of allowing rdtsc to be turned off without breaking clock_gettime. :)

I'll send out a real set of patches in the next few days.  I'll even
try to benchmark them :)

>
> ---
>  arch/x86/kernel/cpu/perf_event.c | 13 ++++++++++++-
>  arch/x86/kernel/process.c        | 24 +++++++++++++++++-------
>  2 files changed, 29 insertions(+), 8 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
> index 16c73022306e..cfc42ff5d901 100644
> --- a/arch/x86/kernel/cpu/perf_event.c
> +++ b/arch/x86/kernel/cpu/perf_event.c
> @@ -1869,6 +1869,17 @@ static ssize_t set_attr_rdpmc(struct device *cdev,
>         return count;
>  }
>
> +void perf_change_rdpmc(bool on, unsigned long *cr4)
> +{
> +       if (x86_pmu.attr_rdpmc_broken)
> +               return;
> +
> +       if (on)
> +               *cr4 |= X86_CR4_PCE;
> +       else
> +               *cr4 &= ~X86_CR4_PCE;
> +}
> +
>  static DEVICE_ATTR(rdpmc, S_IRUSR | S_IWUSR, get_attr_rdpmc, set_attr_rdpmc);
>
>  static struct attribute *x86_pmu_attrs[] = {
> @@ -1928,7 +1939,7 @@ void arch_perf_update_userpage(struct perf_event_mmap_page *userpg, u64 now)
>
>         userpg->cap_user_time = 0;
>         userpg->cap_user_time_zero = 0;
> -       userpg->cap_user_rdpmc = x86_pmu.attr_rdpmc;
> +       userpg->cap_user_rdpmc = x86_pmu.attr_rdpmc && test_thread_flag(TIF_SECCOMP);
>         userpg->pmc_width = x86_pmu.cntval_bits;
>
>         if (!sched_clock_stable())
> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> index e127ddaa2d5a..b74c0400851e 100644
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -201,12 +201,15 @@ void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p,
>                       struct tss_struct *tss)
>  {
>         struct thread_struct *prev, *next;
> +       struct thread_info *pi, *ni;
>
>         prev = &prev_p->thread;
>         next = &next_p->thread;
>
> -       if (test_tsk_thread_flag(prev_p, TIF_BLOCKSTEP) ^
> -           test_tsk_thread_flag(next_p, TIF_BLOCKSTEP)) {
> +       pi = task_thread_info(prev_p);
> +       ni = task_thread_info(next_p);
> +
> +       if ((pi->flags & _TIF_BLOCKSTEP) ^ (ni->flags & _TIF_BLOCKSTEP)) {
>                 unsigned long debugctl = get_debugctlmsr();
>
>                 debugctl &= ~DEBUGCTLMSR_BTF;
> @@ -216,13 +219,20 @@ void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p,
>                 update_debugctlmsr(debugctl);
>         }
>
> -       if (test_tsk_thread_flag(prev_p, TIF_NOTSC) ^
> -           test_tsk_thread_flag(next_p, TIF_NOTSC)) {
> +       if ((pi->flags & (_TIF_NOTSC | _TIF_SECCOMP)) ^
> +           (ni->flags & (_TIF_NOTSC | _TIF_SECCOMP))) {
> +               extern void perf_change_rdpmc(bool, unsigned long *);
> +               unsigned long cr4 = read_cr4();
> +
>                 /* prev and next are different */
> -               if (test_tsk_thread_flag(next_p, TIF_NOTSC))
> -                       hard_disable_TSC();
> +               if (ni->flags & _TIF_NOTSC)
> +                       cr4 |= X86_CR4_TSD;
>                 else
> -                       hard_enable_TSC();
> +                       cr4 &= ~X86_CR4_TSD;
> +
> +               perf_change_rdpmc(!(ni->flags & _TIF_SECCOMP), &cr4);
> +
> +               write_cr4(cr4);
>         }
>
>         if (test_tsk_thread_flag(next_p, TIF_IO_BITMAP)) {



-- 
Andy Lutomirski
AMA Capital Management, LLC
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